This invention relates to automatic flow control valves that maintain a desired output flow rate in response to a varying pressure differential across the valve.
The output flow rate (Q) of a valve is a function of the effective area (A) of the ports of the valve, the fluid pressure differential across the valve (.DELTA.p), and the shape of the flow path through the valve (s), such that: ##EQU1##
Automatic flow control valves typically maintain a constant flow rate (Q) by modifying the area (A) of the ports inversely to a change in the pressure differential (.DELTA.p) across the valve. For example, the valves may have a fixed-area output port and a set of adjustable input ports. In response to an increase in the pressure differential across the valve, the input ports move relative to an occluding element that partially blocks the ports, and thus reduces their effective area. The dimensions of the valve are carefully controlled to ensure that the occluding element reduces the effective area (A) of the input ports in response to an increase in the pressure differential (.DELTA.p) in a manner to maintain a constant output flow rate.
In one type of automatic flow control valve, described in U.S. Pat. No. 5,054,516 (Okerblom), incorporated by reference, a cup with a set of input ports is placed coaxially inside one end of a hollow valve housing. The cup fits inside a pilot ring that serves as the occluding element. The housing has a fixed-size output port at an opposite end. One end of a spring biases a flange on the cup against a portion of the housing that supports the pilot ring. An opposite end of the spring is connected to an end cap. Prior to use, the load on the spring must be precisely adjusted by twisting the end cap to a predetermined torque.
In use, fluid flows through the input ports on the cup and out from the output port of the valve. As the pressure differential across the valve increases and produces a force on the cup that exceeds the spring preload, the cup begins to move along the housing and inside the pilot ring. The pilot ring begins to occlude the input ports on the cup and reduces the effective area of the input ports in a manner to maintain a constant output flow rate.
Prior to use, the constant output flow rate of the valve can be adjusted by nesting the cup within a slightly larger cup, also having a set of input ports. The amount of overlap between the set of ports on each cup determines the maximum area (A) of the input ports that will be exposed to fluid flow. The constant flow rate (Q) of the valve can therefore be adjusted by rotating the cups relative to one another to vary the amount of overlap between the sets of ports.
In another design, described in U.S. Pat. Nos. 3,752,184 and 3,752,183 (Griswold), the valve housing is a cartridge formed from a sheet-metal cylinder. Each end of the cylinder is bent in radially, to provide an aperture for fluid flow that is smaller than the cylinder radius. A cup having a set of input ports distributed on its side surface extends out from one aperture of the cartridge. The cup is biased against fluid flow by a spring placed longitudinally within the cartridge. The valve attempts to maintain a constant output flow rate in the manner described above, with the bent-over end of the cartridge acting as the occluding element and partially blocking the input ports on the cup as the cup slides into the cartridge in response to an increase in the pressure differential across the valve.
In both the Okerblom and Griswold valves, there are difficulties in achieving accurate and repeatable flow characteristics. In particular, it is difficult to control the pressure differential at which the -occluding element begins to block the input ports of the valve. As a result, the effective input port area of the valve may not vary in the proper manner with the pressure differential across the valve, causing an unreliable output flow rate.
In the Griswold valve, the pressure at which occlusion begins depends on the length of the cartridge housing, because one end of the cartridge serves as the occluding element. This length is difficult to control during manufacturing because the cartridge is generally manufactured by bending the ends of a metal cylinder to form the aperture at each of its ends. As a result, it becomes difficult to precisely control the pressure at which the cup will begin to move into the cartridge and the end of the cartridge occlude the input ports.
In the Okerblom valve, the pressure differential at which occlusion begins is sensitive to the preload exerted by the spring against the cup. As a result, the spring preload must be precisely adjusted prior to use.
Another automatic flow control valve, described in
U.S. Pat. No. 4,074,693 (Kates), contains a complex fluid path which is partially transverse to the longitudinal axis of the valve.